The present invention generally relates to an ion source used in an ion implanter and more particularly, relates to an indirect hot cathode (IHC) ion source used in an ion implanter that does not have arcing problems caused by ion deposition at near the electrode.
Ion implantation method has been used for placing impurity, or doping ions in a semiconductor material such as in a silicon substrate at precisely controlled depths and with accurate control of dopant ion concentration. One of the major benefits of the method is its capability to precisely place ions at preselected locations and at predetermined dosage. It is a very reproducible process that enables a high level of dopant uniformity. For instance, a typical variation of less than 1% can be obtained across a wafer.
An ion implanter operates by providing an ion source wherein collisions of electrons and neutral atoms result in a large number of various ions being produced. The ions required for doping are then selected out by an analyzing magnet and sent through an acceleration tube. The accelerated ions are then bombarded directly onto the portion of a silicon wafer where doping is required. The bombardment of the ion beam is usually conducted by scanning the beam or by rotating the wafer in order to achieve uniformity. A heavy layer of silicon dioxide or a heavy coating of a positive photoresist image is used as the implantation mask. The depth of the dopant ions implanted can be determined by the energy possessed by the dopant ions, which is normally adjustable by changing the acceleration chamber voltage. The dosage level of the implantation, i.e. the number of dopant ions that enters into the wafer, is determined by monitoring the number of ions passing through a detector. As a result, a precise control of the junction depth planted in a silicon substrate can be achieved by adjusting the implantation energy, while a precise control of the dopant concentration can be achieved by adjusting the dosage level.
A schematic of a conventional high energy ion implantation apparatus 10 is shown in FIG. 1. In the ion implanter 10, an ion source 20 is utilized in which collisions of electrons and neutral atoms result in a large quantity of various ions. The ions required for doping are then selected out by an analyzing magnet 12 and sent through an acceleration tube 14 that are then accelerated again by a high energy accelerator 16 equipped with an electron stripper and a magnet 18 to bombard a wafer 22 mounted on a mechanically scanned coned disc 24. The coned disc 24 has a capacity of 17 wafers for mounting on its surface and for scanning each wafer upon rotation of the disc 24. The high energy accelerator 16 operates at a high voltage, i.e. normally in a range between about 150 kV and about 750 kV. The coned disc 24 can be preprogrammed to tilt the wafers 22 mounted thereon at an implant angle between xe2x88x92100 to +10xc2x0. A usual implantation time required for each wafer is about 20 min.
A detailed cross-sectional view of the ion source 20 of FIG. 1 is shown in FIG. 2. The ion source 20 is constructed by a chamber 26 which includes a gas inlet 28 for feeding a reactive gas into the chamber cavity 30. A small quantity of a gas is passed through a vaporizer oven and then into the ion source chamber 30 which includes a cathode 40 and an anti-cathode 42. The cathode 40 further includes a heated filament 44 and a filament shield 46. The filament 44 is heated by a filament power supply 48 while the filament shield 46 is connected to a bias power supply 50. The ion source chamber 20 is further powered by an arch power supply 52 and a pre-acceleration power supply 54.
The ion source 20 can be operated in the following manner. First, the filament 44 is heated by passing electric current through it, derived from the power supply 48. The heating of the filament causes thermionic emission of electrons from the surface of the filament. An electric field, typically of a magnitude between 30 and 150 volts is applied between the filament and the chamber walls using the arc power supply 52. The field accelerates the electrons in the filament area to the chamber wall. A magnetic field is then introduced that is perpendicular to the electric field and causes the electrons to spiral outward, increasing the path length and chances for collisions with the gas molecules. The collisions break apart many of the molecules and ionize the resultant atoms and molecules by knocking outer shell electrons out of place. As charged particles, these atomic or molecular ions can now be controlled by magnetic and/or electric fields. Source magnets are employed to change the ion path from a straight path to a helicoid path. With one or more electrons missing, the particles carry a net positive charge. An extraction electrode, i.e. the anti-electrode, is placed in proximity to a slit and held at a negative potential attracts and accelerates the charged particles out of the chamber through the slit opening 32 provided in a sidewall 34 of the chamber 26. Ions 36 existing the chamber cavity 30 are passed through an acceleration tube 14 (FIG. 1) where they are accelerated and through the high energy accelerator 16 to the implantation energy as they move from high voltage to ground. The accelerated ions form a beam that is collimated by a set of apertures (not shown). The ion beam is then scattered over the surface of a wafer 22 mounted on the coned disc 24.
In the conventional ion source 20 shown in FIG. 2, after operation over a period of time, the processing of gases in the chamber cavity 30 results in the accumulation of materials 38 deposited from the gases. The material accumulation is especially severe at vicinities 56 that is close to the filament shield 46. Since the gap 58 provided in-between the filament shield 46 and the endwall 60 is usually very small, i.e. in the range of 1 mm to avoid the escape of plasma ions, the gap 58 is easily filled with the deposited materials and causing either arcing or electrical shorting between the chamber wall 60 and the filament shield 46. The arcing or electrical shorting around the filament shield 46 can cause serious machine malfunction by stopping the generation of plasma ions inside the ion source cavity.
It is therefore an object of the present invention to provide an indirect hot cathode ion source for an ion implanter that does not have the drawbacks or shortcomings of the conventional ion sources.
It is another object of the present invention to provide an indirect hot cathode ion source that does not have arcing or electrical shorting problems between a cathode and a chamber wall.
It is a further object of the present invention to provide an indirect hot cathode ion source that utilizes a cathode including a filament shield that does not have shorting or arcing problems with the chamber wall to which the shield is in close proximity.
It is another further object of the present invention to provide an indirect hot cathode ion source that is equipped with a cathode including a filament shield mounted in close proximity to an inner periphery of an opening in an endwall equipped with a torroidal-shaped recess adjacent to the filament shield for avoiding shorting or arcing.
It is still another object of the present invention to provide an indirect hot cathode ion source that is equipped with a filament shield spaced apart from an inner periphery of an opening in an endwall by a distance of at least 2 mm.
In accordance with the present invention, an indirect hot cathode ion source equipped with a filament shield spaced apart from an inner periphery of an opening in a chamber wall by a distance of at least 2 mm such that arcing or electrical shorting does not occur is provided.
In a preferred embodiment, an indirect hot cathode ion source is provided which includes a chamber formed by two endwalls, two sidewalls, a top and a bottom wall defining a cavity therein for producing plasma ions, an opening through one sidewall of the chamber for ejecting plasma ions therethrough, an anode situated inside the chamber positioned in close proximity to a first endwall of the chamber, and a cathode situated inside the chamber positioned in close proximity to a second endwall opposing the first endwall of the chamber.
The cathode further includes: a filament for passing an electrical current there through and a filament shield of cylindrical shape surrounding the filament spaced apart from an inner periphery of an opening in the second endwall, the inner periphery of the opening in the second endwall is provided with a torroidal-shaped recess in and along an inner periphery of the opening adjacent to the cavity of the chamber such that deposition of materials on the inner periphery of the opening and electrical shorting with the filament shield are avoided.
The indirect hot cathode ion source may further include a gas inlet through the chamber wall for feeding a gas into the chamber cavity. The torroidal-shaped recess in the inner periphery of the opening may have a width between about 1 mm and about 5 mm, and preferably between about 1 mm and about 3 mm. The filament housing of cylindrical shape may have a diameter between about 7 mm and about 20 mm, or a diameter preferably between about 8 mm and about 12 mm.
The plasma ions generated in the cavity may include P+, B+ and As+. The chamber may be formed substantially of molybdenum. The inner periphery of the opening in the second endwall may be formed of graphite for high temperature endurance. A gap is formed between the filament shield and the inner periphery of the opening in the second endwall that is less than 2 mm. The first endwall, the two sidewalls and the top and bottom wall may be formed of molybdenum while the second endwall may be formed of graphite.